Fix typo in chunk setting example

[djhoese]

CC @adybbroe @simonrp84

• [x] Closes #2856
• [ ] Tests added
• [x] Fully documented
• [ ] Add your name to AUTHORS.md if not there already

[coveralls]

Pull Request Test Coverage Report for Build 9959805836

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 96.054%

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Files with Coverage Reduction	New Missed Lines	%
satpy/tests/utils.py	2	93.16%
satpy/tests/reader_tests/gms/test_gms5_vissr_l1b.py	3	98.67%
satpy/tests/reader_tests/gms/test_gms5_vissr_navigation.py	3	97.18%
<!--	Total:	8		-->

Totals
Change from base Build 9842330086:	-0.02%
Covered Lines:	51679
Relevant Lines:	53802

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💛 - Coveralls